drm/i915/panel: register drm_panel and call prepare/unprepare for eDP

drm/i915/panel: register drm_panel and call prepare/unprepare for eDP

Allocate and register drm_panel to allow the panel_follower framework to
detect the eDP panel and pass drm_connector::kdev device to drm_panel
allocation for matching.
Call drm_panel_prepare/unprepare in ddi_enable for eDP to allow the
followers to get notified of the panel power state changes.

Note: This is for eDP with DDI platforms only.

v2: remove backlight setup from panel_register (Jani)
v3: Updated the commit message (Jani)

Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250624-edp_panel-v3-1-e8197b6d9fde@intel.com